The present invention relates to an organic semiconductor electrolyte capacitor and a process for producing the same. More particularly, the invention relates to an improved process for forming an electrolyte layer of an organic semiconductor.
Solid-electrolytic capacitors use an anode in a foil or block form made from a film-forming metal such as aluminum or tantalum. The anode is etched to increase its surface area or made porous by sintering a compact of fine particles. The anode surface is overlaid with a dielectric oxide film, which is further coated with an electrolyte layer connected to a conductive cathode extension.
The prior solid-electrolyte layer is conventionally formed of manganese dioxide. Manganese dioxide solid-electrolyte is formed on the dielectric oxide film by impregnating the anode with a manganese nitrate solution and pyrolyzing the manganese nitrate into manganese dioxide at about 300.degree. C. Only a small amount of manganese dioxide is deposited by one cycle of this process, and in order to provide the necessary deposit thickness, up to about 10 cycles or more must be repeated. This is disadvantageous not only because an extremely complicated process is required but also because the dielectric oxide film is deteriorated by the heat or gases produced during the pyrolysis of the manganese nitrate solution.
A proposal has been made for replacing manganese dioxide by a semiconductive organic compound as the material of the electrolyte layer. Complex salts of 7, 7, 8, 8-tetracyanoquinodimethane (hereunder TCNQ) are currently known as organic electrolytes. The TCNQ complex salts are solid at room temperature and require special methods for depositing them on a capacitor element as the electrolyte. According to U.S. Pat. No. 3,214,648, the anode is dipped in a solution having a TCNQ complex salt dissolved in an organic solvent, and after recovering the anode from the solution, the organic solvent is evaporated to disperse the TCNQ complex into a layer forming on the anode surface. However, the solvent used in this method has a low concentration of TCNQ complex salt and, as in the case of the manganese dioxide layer, up to about cycles of impregnation are required to deposit an acceptable thickness of the TCNQ complex salt layer. Therefore, the problem of process complexity still remains in the technique proposed by U.S. Pat. No. 3,214,648.
Japanese Patent Publication No. 32303/1976 proposes depositing on the anode surface a dispersion comprising a polymer compound and fine particles of a TCNQ complex salt. However, as in the method shown in U.S. Pat. No. 3,214,648, the TCNQ complex salt is crystallized or dispersed in the crystalline state after evaporation of the solvent, and because of the incomplete contact between the complex salt and the dielectric oxide film on the anode (which has been etched to provide a complex and irregular surface profile), the desired electrostatic capacity is not obtainable between the anode and cathode.
Another method has recently been propsed for depositing a TCNQ complex salt on the anode. This method consists of first heating the complex salt alone to a temperature higher than its melting point, then dipping the anode in the resulting melt, and recovering and cooling the anode. According to this method, the TCNQ complex deposited on the anode has such a high concentration that the desired amount of the TCNQ complex salt can be deposited by a single cycle of impregnation. However, the TCNQ complex salts are vulnerable to heat and many of them substantially preclude the application of this method since they are decomposed upon a very short period of heating above their melting points. For example, if isopropyl-isoquinolinium TCNQ complex salt is heated above 200.degree. C., it smokes before melting and decomposes to form an insulator. In order to avoid this problem, the impregnation step must be completed within a short time and followed by rapid cooling. However, such rapid treatments require a complex apparatus and are unable to achieve a high yield. A further problem is caused by the fact that after cooling, the TCNQ complex salt is crystallized and the resulting poor contact with the dielectric oxide film causes insufficient electrostatic capacitance.